The invention relates to machine elements and mechanisms, but more particularly, the invention relates to gearing with a laminated, plural driving surface V-belt.
There are several constructions of endless power transmission belts or so called V-belts in current use. The belts are used in many applications where they operate around various size sheaves at high tensile loads. It is important that the belts have good transverse rigidity to support the cords of the tensile section without significantly impairing its flexibility for bending around various diameter sheaves. The majority of belts in use today support the load carrying tensile cord with a first elastomeric layer inwardly of the cord that is thicker than a second elastomeric layer that is positioned outwardly of the tensile cord. The inner elastomeric layer is subject to compression forces while the outer elastomeric layer is subjected to tension forces when the belt is operated around sheaves. The first or inward layer is oftentimes referred to as the "undercord" or "compression section" while the second or outward layer is referred to as the "overcord" or "tension section."
The undercord must have sufficient modulus to support the tensile section of cords so that the belt cross-section does not significantly deform as the opposing side edges of the belt are forced against the flanges of a sheave. In reality, the undercord does not perfectly support the tensile cords as the belt enters the sheaves. There is deformation of the belt which places a higher stress level on those cords at the edge of the belt. After the belt leaves the sheave in its endless path, the belt cross-section resumes its normal shape. consequently, the undercord must have sufficient rigidity to adequately support the tensile cords as they are continuously cycled between high and low stress levels upon entry and exit from a sheave. A V-belt bends or flexes about its neutral axis which is primarily, if not completely, defined by the cords of the tensile section. Consequently, the undercord of a belt is cycled to compression and the overcord of the belt is cycled to tension upon its entry into a rotating sheave.
Generally speaking, rubber has better flex fatigue characteristics when cycled in compression than it does when it is cycled in tension. This is but one of many reasons why present belts have an overcord that is substantially thinner than the undercord. For example, the undercord thickness may comprise 78 percent of the total belt thickness whereas the overcord would comprise only 22 percent of the total belt thickness. This allows the overcord to be stretched to a degree that is less than the degree to which the undercord is compressed. The difference in degree of stretch somewhat compensates for the lower fatigue life of the rubber in tension in the overcord.
Two processes are in general use today for making belts. The first process involves shaping the belt by skiving into its trapezoidal or "V-section" prior to curing or vulcanization. Materials skived from the edges may be economically reprocessed.
The belt making process of the second type involves curing a sleeve and then shaping individual belts to a V-profile. The skivings between belts cut from a sleeve are fully cured and therefore are not readily reusable.
One way to eliminate the waste of the cured skivings is to build a belt sleeve with a center cord line and then cut the belts in a manner as disclosed by Lejeune in U.S. Pat. No. 2,153,966. Every other belt cut from the sleeve is inverted for proper upright orientation. Inversion slightly pre-tensions the overcord and pre-compresses the undercord. Cutting the sleeve in such a manner is an expedient to avoid waste. However, a belt so produced has an overcord thickness that is substantially equal to the undercord thickness. As brought forth above, a thick overcord introduces fatigue problems while a thin undercord yields less material for transversely supporting the cords of the tensile section.
Some early individually molded belts such as disclosed by Delzell in U.S. Pat. No. 1,432,973 are made by spirally wrapping several layers of rubberized fabric together to form a sleeve. Uncured belts also are cut from the sleeve in a manner requiring every other belt to be inverted to an upright position. The belts are stretched prior to vulcanization to relieve compression of the inner peripheries. While such belts are homogeneous, they are not suited for more modern high power transmission applications because they lack a high modulus tensile section with transverse support means. Also, such belts flex about an undetermined axis whereas belts with a high modulus tensile section flex about the axis of the tensile section. Flexing about an undetermined axis introduces problems with undercord and overcord fatigue.
Thus, the problem associated with such belts is to have the undercord and overcord of a symmetrical construction so that an invertedly cured belt has substantially the same physical characteristics as an adjacently cut and cured upright belt. Summarily, prior art belts have problems primarily with premature overcord fatigue.